Silk tents and spacesuits

Arriving at the office. Photo: Annemarie Nagle

This post was co-written by Frank Lab manager Annemarie Nagle and PhD student Kristi Backe, who is currently studying the pine processionary moth in France. Annemarie recently traveled to France to help Kristi install temperature loggers at field sites around the country. Read more about the project HERE.

It’s winter in the northern hemisphere, and for many of us, that means piling on lots of layers, cranking up the thermostat, and microwaving the water in our water bottle before we drink it. (Okay, maybe that last one is just for extreme cold-haters like Annemarie, but you get the point.) We’re all taking measures to stay toasty this time of year.

But what do our ectothermic friends do when the temperature falls? Many insects deal with winter by riding it out as pupae or eggs, “resting” life stages in which the insect doesn’t move around or eat. The pine processionary moth (PPM; Thaumetopoea pityocampa), on the other hand, spends winter as a caterpillar that must stay warm enough to feed and grow during the cold months.

Tent caterpillar nests are a common sight along roadsides in the U.S. In large parts of Europe, the most noticeable nests in trees are made by the caterpillars of the pine processionary moth. Photo: Kristi Backe

PPM caterpillars live in big groups (“colonies”) inside silk tents that they build in pine trees. The caterpillars hang out in these nests during the day, only emerging at night to feed on the tree’s needles. As temperatures get colder, the caterpillars add more layers of silk to the tent, eventually making a thick, tough shelter that is protection not only from predators but also from the elements. Generally, the caterpillars will build their tent on the edge of the branches in the sunniest part of the tree to maximize the sun’s warming effects. This turns out to be an effective strategy, and it can be up to 20°C warmer inside the tent on a sunny day than it is outside.

Having a cozy tent is important for the obvious reason that freezing to death is bad, and at the northern part of the PPM’s range, near Paris, freezing to death is a real possibility. But the nest is also important because the caterpillars have very specific temperature thresholds that determine if they can feed. They can only feed on nights where (1) nighttime air temperatures are above freezing (0°C) and (2) the caterpillars experienced temperatures above 6°C during the previous day. This 6°C mark is called the “activation temperature,” and if the caterpillars haven’t been activated, they won’t feed at night even if the nighttime temperature is above freezing. Thanks to their sun-exposed tent, the caterpillars often experience daytime temperatures above the activation threshold even when the outdoor temperature is too cold. In other words, because their nest is super warm, the caterpillars are able to feed on nights when they otherwise couldn’t.

Caterpillars might also get a boost by living in cities. Cities are typically warmer than surrounding forests, and this could mean that city-dwelling caterpillars reach the activation temperature more often. At night, when the temperature difference between cities and forests is the greatest, caterpillars in urban areas might spend more time above 0°C, giving them extra time to feed. With more opportunities for feeding, city caterpillars may grow faster and be larger, making them more resilient to the really cold temperatures that come along later in the winter.

In order to test hypotheses about how PPM caterpillars in the city compare to ones in forests, it’s important to understand exactly what temperatures the caterpillars are experiencing. This means we need a temperature data logger inside the nests.

On the surface, this doesn’t sound like an onerous task, but these caterpillars are pretty pesty. Each larva is covered in patches of barbed, skin-irritating hairs that they can shoot off at any moment if disturbed. So unless you want an itchy rash, your best bet is to steer clear of these little critters altogether. And if you’re a couple of scientists trying to insert a data logger into a nest filled with two hundred of them? Well, your best bet is to suit up in some protective gear.

We hung temperature loggers outside the nests too, but that was the easy part. Photo: Alain Roques

A PPM nest after the caterpillars have sealed the temperature logger inside. You can just make out a corner of the yellow plastic rectangle that holds the logger in the nest. Photo: Alain Roques

Fortunately, it’s usually pretty easy to insert the data logger without majorly disturbing the caterpillars, and they get over it pretty quickly. Within a day, they’ve already patched up the hole in the nest with more silk (sealing the logger inside), and they go about their business. And we moved on to the next field site, taking in the beautiful French countryside along the way.

This is the second in a series of posts about Kristi’s pine processionary moth research. (See the first one HERE.) At the end of the winter, she’ll head back to all of the sites to dig out the loggers and see what the caterpillars have been doing. Stay tuned for an update. Kristi’s project is supported by NSF GROW and a Chateaubriand Fellowship from the Embassy of France in the U.S.